This application is based on Japanese Patent Application Nos. 11-212348, 11-225177, and 11-274594 filed in Japan on Jul. 27, 1999, Aug. 9, 1999, and Sep. 28, 1999, respectively, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a liquid crystal display apparatus and its driving method, and more particularly, to a liquid crystal display apparatus equipped with a liquid crystal display using liquid crystal having a memory capability, and to the driving method for said apparatus.
2. Description of the Related Art
Using a conventionally known liquid crystal display in which a liquid crystal that exhibits a cholesteric phase at room temperature, such as a cholesteric liquid crystal or a chiral nematic liquid crystal, is sandwiched between two substrates, display may be performed by alternating the state of the liquid crystal between a planar state and a focal conic state.
In other words, when the liquid crystal is in a planar state, where the helical pitch is deemed (P) and the average refractive index is deemed (n), light having the wavelength xcex=Pxc2x7n is selectively reflected. Where the liquid crystal is in a focal conic state, when the selective reflection wavelength of the liquid crystal is in the infrared range, the liquid crystal scatters the light, and when the selective reflection wavelength is in the range shorter than the infrared range, the liquid crystal allows visible light to pass through. As a result, by setting the selective reflection wavelength to be in the visible range, and by locating a light-absorbing layer on the side of the element opposite the side that is observed, display of the selectively reflected color may be obtained when the display is in the planar state, while black is displayed when the display is in the focal conic state.
Where the selective reflection wavelength is set to be within the infrared range and the light-absorbing layer is located on the side of the element opposite the side that is observed, a black display is obtained because light having a wavelength within the infrared range is reflected but light in the visible range passes through when the liquid crystal is in the planer state. Consequently, a white display can be obtained through scattering of the light when the display is in the focal conic state.
By using three stacked elements set to selectively reflect red, green and blue, respectively, a color display may be obtained.
This type of liquid crystal display may be alternated between a planar state and a focal conic state through the application of voltage. If the threshold voltage required to eliminate the twist in the liquid crystal is deemed Vth1, when Vth1 is applied for a sufficient amount of time, and then the voltage is reduced to a lower voltage Vth2, the display enters a planar state. When a voltage between Vth2 and Vth1 is applied for a sufficient amount of time, the display enters a focal conic state. These two states remain stable even after the application of voltage is stopped. It is also known that these two states may coexist, enabling halftone display.
Incidentally, the display state of the liquid crystal generally depends on the ambient temperature. Chiral nematic liquid crystal in particular has a temperature characteristic in which the display state (the Y value, i.e., the luminous reflectance) changes in accordance with the surrounding temperature. This is caused mainly by the fact that the viscosity of the chiral nematic liquid crystal falls as its temperature rises. Therefore, when the display is driven by means of a pulse voltage having a constant voltage level and pulse width at all times, chiral nematic liquid crystal entails the problem that its display state changes depending on the temperature.
With ordinary nematic liquid crystal, the drive voltage must be continuously applied to maintain the display, and real-time ambient temperature information from a temperature detection unit must be incorporated as a drive condition. However, the incorporation of this real-time temperature information imposed a substantial burden on the control unit (i.e., the CPU), and entails high power consumption. On the other hand, when the liquid crystal used has a memory capability that can maintain a display even if the application of drive voltage is stoppedxe2x80x94such as cholesteric liquid crystal or chiral nematic liquid crystalxe2x80x94the proper timing and frequency of the incorporation of the temperature information have not yet been determined.
In addition, chiral nematic crystal is known to have a unique hysterisis phenomenon. Therefore, in order to avoid the occurrence of problems arising due to this hysterisis phenomenon, when performing driving, it is desired that the desired pixels be set to the desired state after a first reset pulse signal is applied to the liquid crystal and that the liquid crystal be reset to the homeotropic state. However, because the reset pulse signal to reset the liquid crystal to the homeotropic state as described above requires more energy than the selection pulse signal used to set the liquid crystal to the desired reflection state, multiple pulse signals that entail different amounts of energy are normally required to drive chiral nematic liquid crystal. Therefore, temperature compensation must be performed for each of these pulse signals, and the problem arises that the driving method and drive circuit become more complex.
Therefore, the object of the present invention is to provide a layered liquid crystal display apparatus that can avoid display state variation regardless of changes in the surrounding temperature.
Another object of the present invention is to provide a liquid crystal display apparatus and associated driving method in which good display can always be performed regardless of changes in the surrounding temperature and in which the driving method and drive circuit are simplified.
Yet another object of the present invention is to provide a liquid crystal display apparatus and associated driving method that reduce the burden on the control unit and efficiently incorporate temperature information while reducing power consumption.
In order to attain these and other objects, the liquid crystal display apparatus reflecting a first aspect of the present invention comprises: a liquid crystal display including a liquid crystal material having a memory capability; a temperature detection unit that detects a temperature of the liquid crystal display or a temperature of the environment surrounding the liquid crystal display; and a control unit connected with the liquid crystal display and the temperature detection unit, the control unit applying drive pulse signals to the liquid crystal display to draw a first image on the liquid crystal display and leaving the liquid crystal without applying a drive pulse signal to maintain the first image by using the memory capability of the liquid crystal, wherein the control unit incorporates temperature information from the temperature detection unit before the drawing of the first image.
The liquid crystal display apparatus described above may also include (i) a first display mode under which the control unit applies the drive pulse signals to the liquid crystal display to draw the first image on the liquid crystal display and leaving the liquid crystal without applying a drive pulse signal to maintain the image by using the memory capability of the liquid crystal; and (ii) a second display mode under which the control unit successively draws a second image to an n-th image data on the liquid crystal display. In this case, when the first display mode is active, the control unit incorporates temperature information from the temperature detection unit before the drawing of the first image, while when the second display mode is active, the temperature information is incorporated by the control unit before the drawing of the second image, and thus incorporated temperature information is commonly used for the drawings of the second image to the n-th image.
In other words, because the temperature information need not be continuously incorporated in the liquid crystal display apparatus described above, power consumption may be reduced by putting the CPU to sleep during periods when redrawing is not being performed, or by putting to sleep all components of the CPU other than those which are necessary to detect instructions to redraw the display.
The liquid crystal display apparatus reflecting a second aspect of the present invention comprises: a liquid crystal display including a liquid crystal material having a memory capability; a temperature detection unit that detects a temperature of the liquid crystal display or a temperature of an environment surrounding the liquid crystal display; and a control unit connected with the liquid crystal display and the temperature detection unit, the control unit applying a first reset pulse signal to the liquid crystal display, the first reset pulse signal being for setting the liquid crystal to a homeotropic state before the liquid crystal is set to the desired selective reflection state, wherein the control unit keeps a voltage level and a pulse width of the first reset pulse signal constant regardless of the temperature detected by the temperature detection unit.
In other words, if the first reset pulse signal is set to a certain fixed pulse width and voltage level, the liquid crystal may be set to the homeotropic state. Therefore, by keeping the pulse width and voltage level of the first reset pulse signal fixed regardless of changes in the temperature, the driving method and the drive circuit may be simplified.
In the liquid crystal display apparatus described above, a selection pulse signal that sets an area of the liquid crystal to a desired state may be applied to drive the crystal after the first reset pulse signal is applied, and then at least one of a voltage level and a pulse width of the selection pulse signal may be changed in accordance with the detected temperature. In other words, because the selection pulse signal has a temperature dependence unique to the liquid crystal, by changing at least one of the voltage level and the pulse width of the selection pulse signal in accordance with changes in temperature, a stable, high-quality display may be obtained.
In the liquid crystal display apparatus described above, where the selection pulse signals are applied after the application of a second reset pulse signal that follows the first reset pulse signal and sets the liquid crystal to the focal conic state, it is acceptable if at least one of a voltage level and a pulse width of the second reset pulse signal is changed in accordance with the temperature detected by the temperature detection unit, or if the voltage level and pulse width are kept constant regardless of the temperature detected by the temperature detection unit.
In other words, the temperature dependence of the second reset pulse signal varies depending on the type of liquid crystal. If the temperature dependence can be ignored when a specific pulse width and voltage level are set, the pulse width and voltage level should be kept constant regardless of changes in the temperature. However, where the liquid crystal exhibits marked temperature dependence, the pulse width and voltage level should vary in response to changes in temperature.
By employing the structure described above, the problems of (i) variation in the reset state of the liquid crystal due to changes in temperature, and (ii) fluctuating display states, may be prevented from occurring, and a stable, high-quality display may be obtained at all times.
The liquid crystal display apparatus reflecting a third aspect of the present invention comprises: a liquid crystal display comprising a plurality of liquid crystal display layers stacked each other; a drive unit connected with the liquid crystal display, the drive unit applying a pulse voltage to each of the liquid crystal display layers to drive the liquid crystal display layers; a temperature detection unit that detects a temperature of the liquid crystal display or a temperature of an environment surrounding the liquid crystal display; and a controller connected with the drive unit and the temperature detection unit, the controller performs a temperature compensation by adjusting at least one of a voltage level and a pulse width of the pulse signal applied from the drive unit to at least one of the liquid crystal display layers based on the temperature detected by the temperature detection unit.
The temperature characteristic of a specific liquid crystal, i.e., the change in the display state (Y value) of the liquid crystal in response to changes in temperature, based on such a parameter as the voltage level and/or the pulse width of the drive pulse voltage, may be predicted beforehand. As a result, by detecting the temperature of the liquid crystal display or the surrounding temperature, temperature compensation in which the drive pulse voltage is adjusted is performed in the present invention. In this way, a fixed display state may be continuously maintained regardless of changes in the ambient temperature.
The controller may perform temperature compensation for all the liquid crystal display layers, or only for specific layers. When temperature compensation is performed for all layers, more precise temperature compensation must be performed, while control is easier to perform when temperature compensation is carried out for only specified layers.
The controller may have separate temperature compensation data for each display layer, or may use common temperature compensation data for all layers. In the former case, more precise temperature compensation must be performed, while in the latter case, the control process is simpler.
In each of the liquid crystal display apparatuses reflecting the first to third aspects of the invention, the temperature detection unit may have multiple sensors, so that the temperature of the liquid crystal display or the temperature surrounding the liquid crystal display is detected by these multiple sensors, and the temperature information from these sensors is reflected in the subsequent temperature compensation. In this case, the multiple sensors may be located at both the observation side and at the back of the liquid crystal display, and may be located at multiple locations in the same plane as the screen of the liquid crystal display.